1. Field of the Invention
The present invention relates to a thin film circuit board and its manufacturing process, wherein the circuit board is formed so that the periphery of a conductor layer arranged on an insulator layer is covered by an insulator having a low dielectric constant.
2. Description of the Related Art
In response to increasing speeds of electronic apparatuses such as mainframe computers, in recent years, the signal propagation speed of conductor layers in circuit boards used in these electrical apparatuses has been made increasingly faster.
Since the signal propagation speed of the conductor layer in these circuit boards is subject to the dielectric constant effect of the insulator layer that supports this conductor layer, it is desirable that the insulator layer be formed by an insulator having a low dielectric constant in order to increase the signal propagation speed.
An example of the prior art is illustrated in the schematic drawing of the prior art of FIGS. 5 and 6a-6e. FIG. 5 is a lateral cross-sectional view, while FIGS. 6a through 6e are drawings of successive steps in the manufacturing process.
As indicated in FIG. 5, a thin film circuit board 9 is formed by arranging conductor layer 1, composed of a thin film that propagates signals and two insulator layers 10 and 11 formed between ground patterns 12 and 13. In addition, the dielectric constant of conductor layer 1 is was minimized as much as possible by providing an insulator layer 5 having a low dielectric constant, around the periphery of conductor layer 1.
When forming this type of thin film circuit board 9, a prescribed pattern is first formed on upper layer 10A of insulator layer 10 composed of a polyimide material deposited on ground pattern 12, as indicated in FIG. 6a. For example, a conductor layer composed of a good conducting material, such as aluminum or copper having a thickness of several .mu.m is formed, followed by etching of insulator layer 10. As a result, undercut portion 6, having a width H1 and a depth H2, is formed between conductor layer 1, on both sides thereof and upper surface 10A of insulator layer 10, as indicated in FIG. 6b.
Next, as indicated in FIG. 6c, a coating of a low dielectric constant insulator 5, such as Teflon and so forth, is formed around the periphery of conductor layer 1, and conductor layer 1, coated with low dielectric constant insulator 5, is deposited so as to be covered by polyimide insulator layer 11 as indicated in FIG. 6d.
Finally, as indicated in FIG. 6e, ground pattern 13 is formed on upper surface 11A of insulator layer 11, thereby resulting in the formation of thin film circuit board 9.
Thus, as a result of nearly entirely covering the periphery of conductor layer 1 with low dielectric constant insulator 5, the dielectric constant of conductor layer 1 is minimized, as much as possible.
However, although it is possible to increase the size of the undercut portion 6 in the direction of depth H2 as indicated in FIG. 6b of lengthening the etching time, there is a limit on increasing that size in the direction of H1, since it is not possible to make said direction of H1 larger by lengthening the etching time.
Thus, when simply performing an etching, the value of width H1 decreases and the surface area of conductor layer 1, that low dielectric constant insulator 5 covers, also decreases. In actuality, the reduction in the dielectric constant of conductor layer 1 is lowered, thus resulting in a reduced increase in signal propagation speed, which is a problem.